1. Field of the Invention
This invention relates to an optical recording medium in which recording and/or reproduction of information signals occurs in a direction along a recording track. This invention also relates to a master disc for producing such optical recording medium.
2. Description of Related Art
An optical recording medium is formed as a disc having a signal recording layer thereon. A light beam is illuminated on this signal recording layer to record and/or reproduce information signals.
Typical of this optical recording medium is a replay-only optical disc in which a row of pits corresponding to information signals to be recorded is pre-formed on a disc substrate, such as a CD (Compact Disc) or CD-ROM (CD-Read Only Memory). In such replay-only optical disc, the major surface of the disc substrate carrying the row of pits operates as a signal recording layer. In the CD, the mark length of each pit constituting the row of pits is a length of the information signals for recording which is normalized by a pre-set bit interval T. That is, the mark length of each pit of the row of pits is an integer number times the bit interval T, with the mark length of the shortest pit being e.g., three times the bit interval (3T).
For example, an optical disc used in a so-called Compact Disc recordable system to allow for post-writing of information signals is also being put to practical use. This optical disc is referred to below as CD-R. The CD-R has a signal recording layer for recording information signals formed of an organic dye-based material and, if irradiated with a light beam, has its reflectance changed at this illuminated location to effect recording. The signals recorded on the CD-R are reproduced by detecting the reflectance of the signal recording layer.
As an optical recording medium, a magneto-optical disc for recording and/or reproducing the information recording layer by exploiting photomagnetic effects, such as a Mini-Disc (MD), and a phase change type optical disc, which allows for rewriting recorded signals by exploiting phase changes of the signal recording layer, such as CD-RW, are also being put to practical application.
In e.g., a CD-R, among the above-mentioned various optical recording mediums, a wobbling groove is formed for extending along a recording track. This wobbling groove is a guide groove formed meandering at a pre-set period. By this meandering of the guide groove, the guide groove itself has signal components. Meanwhile, the guide groove is formed for extending along a recording track to provide for facilitated the tracking servo by e.g., the push-pull method.
In the CD-R, the sector information comprehending the frequency modulated (FM) absolute time information is recorded as signals by this wobbling groove. This sector information so recorded is referred to below as the wobbling signals. That is, in a Compact Disc Recordable System, employing the CD-R as a recording medium, a light beam is converged on the wobbling groove and the reflected light therefrom is detected to detect the wobbling signal having e.g., 22.5 kHz as a carrier wave. This wobbling signal is frequency demodulated to detect the absolute time information in recording and/or reproduction. The system in which the absolute time information is recorded as the wobbling signal is generally termed ATIP (Absolute Time In Pre-groove). There is also a system, termed ADIP (Address in Pre-groove) system, in which the address information is recorded as wobbling signals.
In the CD-R, the information signals, modulated in EFM (8 to 14 modulation) with respect to the recording track, are recorded and/or reproduced, based on the wobbling signals, as these wobbling signals are read out.
In the system in which the sector information inclusive of the absolute time information or the address information is recorded as the wobbling signals, the information signals can be continuously recorded and/or reproduced, in a manner advantageous in maintaining compatibility with respect to an optical disc on which information signals are recorded continuously. In a system in which the sector information inclusive of the absolute time information or the address information is not recorded as the wobbling signals, but are recorded e.g., at the leading end of each sector, the absolute time information or the address information and the information signals for recording are time-divisionally recorded, with the result that the information signals for recording are not continuous signals. This renders it difficult to maintain compatibility with respect to an optical disc on which information signals are recorded continuously, such as CD.
The optical disc in which the wobbling signals are recorded in a pre-groove formed along the recording track may be exemplified by, for example, an MD or a CD-RW, in addition to the CD-R.
In the above-described optical disc, it is strongly desired to increase the recording density to permit more and more information signals to be recorded for the same outer size of the disc as the standardized pre-existing optical disc. As the optical disc with a higher recording density than a conventional optical disc such as a conventional CD or CD-ROM, there have so far been proposed a DVD-R, DVD-RW or DVD+RW.
However, in the DVD-R, DVD-RW or DVD+RW, in distinction from the above-mentioned routine optical discs, exemplified by the CD or CD-ROM, recording signals are recorded not by the EFM modulation but by the EFM+modulation (8 to 16 modulation). The wobbling signals also are not frequency modulated signals, with the modulation frequency of 22.05 kHz, as in the case of the above-mentioned pre-existing optical disc. In the DVD-R or DVD-RW, wobbling signals of the sine waveform of a sole wavelength are used, whereas, in the DVD+RW, pulse modulated wobbling signals are used.
Therefore, if, in a recording and/or reproducing apparatus for recording and/or reproducing information signals on or from the DVD-R or DVD-RW, it is desired to achieve compatibility with respect to the pre-existing CD or CD-ROM, it is necessary to provide modem circuitry for information signals and wobbling signals separately with the result that the system structure is complex and expensive.
Moreover, if, in an optical disc provided with a row of pits corresponding to information signals and which is used for replay only, such as CD or CD-ROM, the row of pits is formed to a high density to achieve high density recording and large recording capacity, and the optical disc is reproduced using a conventional recording and/or reproducing apparatus, there is raised a problem of increased jitter and lowered C/N ratio to render the stable replay operation difficult.
It is therefore an object of the present invention to provide an optical recording medium whereby a high recording density is achieved and recording and/or reproduction can be carried out reliably in stability as compatibility with standardized pre-existing optical recording mediums is kept.
It is another object of the present invention to provide a master disc for the preparation of this sort of the optical recording medium.
In one aspect, the present invention provides an optical recording medium in which information signals are recorded and/or reproduced along a recording track, wherein the recording density is made to be higher than that in a standardized pre-existing optical recording medium despite the fact that the modulation/demodulation system for information signals recorded and/or reproduced on or from the recording track is made to be the same as that for the standardized pre-existing optical recording medium, and wherein, if the track pitch of the recording track is Tp, the shortest recording mark period in recording on the recording track is Mc, the wavelength of a light beam used in recording and/or reproducing the recording track is xcex, and the numerical aperture of the objective lens is NA, the following relation (1) to (3)
X=Tp/(xcex/NA)xe2x80x83xe2x80x83(9)
Y=Mc/(xcex/NA)xe2x80x83xe2x80x83(10)
Y greater than 2.9118X2xe2x88x925.5422X+3.19xe2x80x83xe2x80x83(11)
is met.
With the optical recording medium according to the present invention, high recording density can be achieved as the combination of the track pitch Tp and the shortest recording mark period Mc is varied from that in the standardized pre-existing optical recording medium. Moreover, recording and/or reproduction can be realized reliably in stability as the combination of the track pitch and the shortest recording mark period of the standardized pre-existing optical recording medium is varied from that of the standardized pre-existing optical recording medium. In addition, recording and/or reproduction can be realized reliably in stability even in case a recording and/or reproducing apparatus adapted for recording and/or reproducing a standardized pre-existing optical recording medium is used for recording and/or reproduction.
In another aspect, the present invention provides a master disc for preparation of an optical recording medium employed in the preparation of an optical recording medium in which information signals are recorded and/or reproduced along a recording track. In the optical recording medium, the recording density of the optical recording medium is made to be higher than that in a standardized pre-existing optical recording medium despite the fact that the modulation/demodulation system for information signals recorded and/or reproduced on or from the recording track is made to be the same as that for the standardized pre-existing optical recording medium. If the track pitch of the recording track is Tp, the shortest recording mark period in recording on the recording track is Mc, the wavelength of a light beam used in recording and/or reproducing the recording track is xcex, and the numerical aperture of the objective lens is NA, the following relation (1) to (3)
X=Tp/(xcex/NA)xe2x80x83xe2x80x83(12)
Y=Mc/(xcex/NA)xe2x80x83xe2x80x83(13)
Y greater than 2.9118X2xe2x88x925.5422X+3.19xe2x80x83xe2x80x83(14)
is met.
In the master disc for an optical recording medium, constructed as described above, such an optical recording medium can be produced in which high recording density can be achieved as the combination of the track pitch Tp and the shortest recording mark period Mc is varied from that in the standardized pre-existing optical recording medium. Moreover, in the optical recording medium produced by the present master disc, recording and/or reproduction can be realized reliably in stability as the combination of the track pitch and the shortest recording mark period of the standardized pre-existing optical recording medium is varied from that of the standardized pre-existing optical recording medium. In addition, in the optical recording medium produced by the present master disc, recording and/or reproduction can be realized reliably in stability even in case recording and/or reproduction is performed by a recording and/or reproducing apparatus adapted for recording and/or reproducing a standardized pre-existing optical recording medium.
In the optical recording medium according to the present invention, recording and/or reproduction can be achieved reliably in stability even if a recording and/or reproducing apparatus for performing recording and/or reproduction on or from a standardized optical recording medium is used for recording and/or reproduction, while high recording density can be achieved. So, a high recording density and an increased recording capacity can be achieved as compatibility with respect to the standardized pre-existing optical recording medium is kept. Moreover, since the modulation/demodulation system for information signals and the wobbling signals can be used in common, the production cost of the recording and/or reproducing apparatus can be suppressed even when the optical recording medium of the present invention is used in conjunction with a recording and/or reproducing apparatus configured for reproducing the standardized pre-existing optical recording medium.
With the master disc for the optical recording medium according to the present invention, such an optical recording medium can be produced in which recording and/or reproduction may be achieved reliably in stability, even if a recording and/or reproducing apparatus for recording and/or reproducing a standardized pre-existing optical recording medium is used for recording and/or reproduction, whilst a high recording density can be realized. Thus, if an optical recording medium is produced using the master disc according to the present invention, such an optical recording medium can be produced in which a high recording density and an increased recording capacity can be achieved as compatibility with respect to the standardized pre-existing optical recording medium is kept.